Mode multiplexing optical coupling device

ABSTRACT

An efficient tapered optical fiber bundle along with the method of manufacturing is presented. The tapered fiber bundle is fully fused to an induced shape with no interstitial space between fibers. To minimize fiber deformation and hence the tapered bundle&#39;s loss, the individual fibers are minimally deformed by positioning them in a fixture with predetermined geometry prior to fusion. The bundle could be optionally reshaped after fusion. The tapered bundle could then be used in its original form as a star coupler, or it could be cleaved and coupled to a multimode fiber, a multi-clad fiber, a cladding-pumped fiber, or an optical system to form an optical device. The resulting optical device has improved efficiency and lower loss compared with prior art devices.

FIELD OF THE INVENTION

[0001] The present invention relates generally to optical devices. Moreparticularly, the present invention relates to mode multiplexing opticalcoupling devices, such as mode multiplexing combiners for use with fiberlasers, fiber pumped solid-state lasers, and optical amplifiers, as wellas to optical couplers and splitters based on mode managed opticalcoupling.

BACKGROUND OF THE INVENTION

[0002] Fused coupler technology, wherein optical fibers are bundledtogether, heated, and pulled lengthwise, is commonly used to producecouplers, combiners and splitters for optical communication systems,medical devices and other industrial applications. Generally, a combineris a passive fiber optic coupler in which power from several inputfibers is combined into one output fiber. Conversely, a splitter divideslight from a single input fiber into two or more output fibers. Thecoupler represents the general case where inputs from one or more fibersare mixed and distributed among one or more output fibers.

[0003] Combiners in particular are seeing new applications for diodepumped lasers, including fiber lasers and solid-state lasers, and diodepumped optical amplifiers. They are used to combine multimode opticalpump power from a multiple sources, such as multimode laser diodes, andtransfer the combined pump power into the inner cladding of a multicladfiber or into a multimode fiber. A multiclad fiber typically has a smallcore (that typically transmits a singlemode or a small number of modes)surrounded by an inner cladding layer of lower refractive index andsignificantly larger cross-section that transmits the multimode pumppower. An outer cladding of even lower refractive index causes themultimode pump power to be confined in the inner cladding by totalinternal reflection. The multiclad fiber is used to combine a singlemode(or multimode mode) signal in the core, along with multimode pump powerin the inner cladding, to a separate device which may be used foramplification. These mode multiplexing combiners are typically used withcladding-pumped fibers. Cladding-pumped fibers are a special case ofmulticlad fiber where the multimode light propagates within the core andinner cladding interacting with special dopants (such as rare-earthelements like Er) in the core that absorb the pump photons and radiatephotons at a different wavelength. Under suitable conditions, thespecial dopants in the core cause stimulated or spontaneous emission atthe different wavelength and can operate in the form of a fiber laser oroptical amplifier. Multiclad fibers containing special dopants for thepurpose of lasing or amplification are known as cladding-pumped fibers.

[0004] For any coupler, splitter, or combiner, it is desirable tomaximize the throughput of optical power from any input fiber, throughthe device, and through any output fiber. For convenience, the case of acombiner is further described, recognizing that the same principlesapply equally to splitters and couplers. The throughput depends onefficiently transferring the total brightness from all the input fibersinto a single output fiber having sufficient capacity to carry thecombined brightness. This transfer can be analysed using modal analysis,ray-tracing methods, or by simple matching of input and outputbrightness. Conservation of brightness is based on the LaGrangeInvariant of an optical system and is typically characterized by thequantity etendue, which is the product of the area of illumination timesthe extended solid angle. For a fixed level of optical power, increasedbrightness implies a decrease in etendue. For a step index multimodeoptical fiber, the etendue can be approximated by E=π²/4 NA² D². If sucha step index fiber is tapered, its etendue remains constant while theeffective numeric aperture (NA) increases as the diameter (D) decreases.In this analysis, the NA refers to the maximum angle of light enteringor exiting the optical fiber according to NA=sin(acceptance angle).

[0005] In order to efficiently transfer power between two opticalelements (in this case from an input fiber bundle into an output fiber),two requirements must be satisfied. First, the etendue on the input sideshould be less than or equal to the etendue on the output side,otherwise the coupling efficiency will be limited by E_(out)/E_(in).Second, the areas must be matched at the junction.

[0006] Prior art combiners, such as that described in U.S. Pat. No.5,864,644 to DiGiovanni et al., rely on the tapering process toeliminate interstitial voids between input fibers, and to develop asuitable circular cross-section in the input fiber bundle. However, itis not possible to solely rely on the tapering process to achieve therequirements for low loss combiners, especially when there is a range inthe number of input ports, or if the output fiber is non-circular.

[0007] It is, therefore, desirable to provide an improved opticalcoupling device that substantially eliminates interstitial spacingbetween input fibers, while providing a good cross-sectional matchbetween the input fiber bundle and the output fiber independent of thenumber of fibers bundled together.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to obviate or mitigateat least one disadvantage of previous optical coupling devices, such ascombiners and splitters. It is a particular object of the presentinvention to provide an improved optical coupling device that hasreduced insertion loss in a wide range of combinations of fiber types,sizes, shapes, and number of ports.

[0009] In a first aspect, the present invention provides a taperedoptical fiber bundle. The tapered optical fiber bundle consists of aplurality of optical fibers formed into a fiber bundle with a minimizedencircling radius. The bundle is adiabatically tapered, andheavily-fused into an induced compact shape with fibers minimallydeformed and no interstitial space between the optical fibers. Theoptical fibers can be multimode, singlemode, multiclad, orcladding-pumped, and one or more of the ports may be terminated.Terminated ports are fibers that are included in the bundle only to formpart of the geometric or optical structure, but are subsequently cut-offoutside the fused region using a technique to minimize reflection fromthe cut endface.

[0010] In a further aspect, the present invention provides an opticalfiber device, such as an optical combiner, an optical splitter, anoptical coupler, for use in an optical system such as a cladding-pumpedfiber laser, fiber pumped solid-state laser, or a cladding-pumpedoptical amplifier. The optical fiber device consists of a tapered fiberbundle coupled to a multimode, multiclad, or cladding-pumped opticalfiber, a second tapered fiber bundle, or a bulk optical device (such asa solid-state laser element). The coupling process maximizes thetransfer of power (and signal) from the inputs to the outputs by virtueof the optimized brightness contained in the input bundle.

[0011] In yet another aspect, the present invention provides a method ofmanufacturing the tapered fiber bundle. First, a plurality of opticalfibers, decoated to remove any polymer or metallic coating layers in theregion where they will be heated, are positioned in a predeterminedconfiguration that will result forming a minimized encircling radius.The positioned fibers are then twisted and bundled under controlledtension to result in a fiber bundle with minimized encircling radius. Anadhesive can be used at both sides of the bundle to secure thepositioning. The bundle is then heated and pulled to heavily fuse thefibers, while adiabatically tapering the bundle, into an induced shapewith no interstitial space between minimally deformed fibers. Ifdesired, glass cladding can be fully or partially removed prior tofusing the bundle. Also one or more singlemode (or multimode) fibers canbe incorporated into the bundle at appropriate locations such that theirposition in the resulting tapered bundle corresponds to similarsinglemode (or multimode) cores in an output bundle or multiclad fiberor cladding-pumped fiber.

[0012] To form an optical fiber device according to the presentinvention, the fused and tapered bundle is cleaved at the taperedregion. The cleaved bundle endface is optionally reshaped by fusionsplicing it to an output fiber of appropriate shape, then recleavingagain. The splicing and re-cleaving can be repeated, if desired, untilsurface tension during fusion splicing causes the cleaved end toapproach the desired cross-sectional geometry. The reshaped cleaved endis then coupled to a suitable optical element, such as a single opticalfiber, a second tapered fiber bundle, or a bulk optical device (such asa solid-state laser element). The method of the present invention caninclude pre-tapering of the output fiber, post-tapering of the junctionbetween the tapered fiber bundle and an output fiber or bundle, andre-coating the junction with a coating material, such as polymer ormetallic material.

[0013] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The file of this patent contains at least one drawing executed incolor. Copies of this patent with color drawing(s) will be provided bythe Patent and Trademark Office upon request and payment of thenecessary fee.

[0015] Embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

[0016]FIG. 1 is a schematic representation of a mode multiplexingoptical coupling device according to an embodiment of the presentinvention;

[0017]FIG. 2 shows an exemplary 7-1 tapered fiber bundle according tothe present invention;

[0018]FIGS. 3a-3 c are cross-sections of three multimode fibers bundledaccording to the present invention;

[0019]FIGS. 4a and 4 b are cross-sections of ten multimode fibersbundled according to the present invention;

[0020]FIG. 5 shows exemplary fixtures to secure fiber bundleconfigurations, prior to fusing and tapering, for two to twenty fibers,respectively; and

[0021]FIGS. 6a to 6 f are cross-sections of tapered fiber bundles havingtwo, three, four, eight, ten and sixteen fibers, manufactured accordingto the method of the present invention.

DETAILED DESCRIPTION

[0022] Generally, the present invention provides a mode multiplexingoptical coupling device, and a method of making such a device. Theoptical device, and related fabrication process, have one or more inputfibers that are fused and tapered and can be coupled to one or moreoutput fibers, such that the optimum cross-sectional shape of the fusedregion depends on the specific details of the output fiber or fibers.For ease of description, the following discussion is limited to the caseof a mode multiplexing combiner having a single output fiber of circularcross-section. However, this is not intended to limit the generality ofthe present invention, and it is clearly within the contemplation of theinventors that the present invention can be used with multiple outputfibers, and/or with output fibers having non-circular cross-sections.

[0023] The object of a mode multiplexing combiner is to maximize thetransfer of power by maximizing the number of multimode fibers ofspecified NA and core diameter that can efficiently transfer power intoa multimode, multiclad, or cladding-pumped output fiber of specifieddimensions and NA. To efficiently transfer power from a multifiber inputbundle to an output fiber, the input fibers are ideally bundled tominimize the aggregate etendue, without causing microbending loss in theoptical fibers. A coupling device 20 according to an embodiment of thepresent invention is illustrated schematically in FIG. 1. The couplingdevice 20 consists of a number of fibers 30 formed into a tapered fiberbundle 22, and spliced to an output fiber 24. The absolute minimumaggregate etendue occurs if the input fibers: first have all claddingremoved, second are fused as a bundle to eliminate all interstitialspace between them, and third the fused bundle is in the formed to acircular cross-section. Such a device is problematic to fabricatebecause the transmission loss is high due to the substantial coredeformation. In addition, the structure is weak due to the completeremoval of the cladding layer, which reduces the strength of the glassand prevents the use of a structural adhesive on the unclad fiber.Instead, a practical, optimized configuration and process is required tominimize aggregate etendue of an input fiber bundle. This is achievedby:

[0024] i) selecting an initial geometric configuration of the inputfibers to maximize packing efficiency, such that the fibers areencompassed by the smallest possible circle before fusing. Thisminimizes the deformation of cores when the fiber bundle is tapered andfully fused. The optimum geometric configuration is not always based onhexagonal close-packed geometry.

[0025] ii) fully fusing the fibers to obtain a degree of fusion close toone, thereby eliminating interstitial space between and around thefibers.

[0026] iii) selectively removing some or all of the fiber cladding fromsome or all of the input fibers to further reduce the aggregate bundleetendue, allowing any remaining cladding to amalgamate, and allowing thecores of the fibers to deform by a minimal amount in order to obtain asubstantially circular bundle.

[0027] iv) tapering the fibers with the primary objectives of (a)maintaining adiabatic tapering conditions to avoid power loss in thebundle and (b) promoting radial fusion between the fibers.

[0028] Generally, in the case of polymer clad fiber, the cladding isalways removed because it is not compatible with the fusing process. Inthe case of glass clad fiber, the cladding can be partly removed (forexample, by chemical etching) when specifically required to reduce theetendue of an input bundle below that of the output fiber.

[0029] As used herein, the packing efficiency (PE) is the ratio betweenthe area of a circle circumscribing the fiber bundle before fusing (Ax)and the sum of the area of all the untapered fibers in the bundle (A₀).The object is to optimize the initial fiber orientation to minimize PE.The degree of tapering (DOT) is the ratio of the tapered core diameterto the untapered diameter. The degree of fusion (DOF) ranges from 0before fusion occurs, to 1 when the fibers are completely fused into acircular cross-section. DOF is based on the area of a circlecircumscribing all of the fibers in the bundle (A).

DOF=(Ax−A)/(Ax−A ₀).

[0030] It is preferable to minimize etendue by obtaining a DOF of 1,which requires the fiber bundle to contain no interstitial spaces and tohave a perfectly circular perimeter, after the fusing process and thesubsequent splicing process. The deformation ratio (DR) is the ratio ofthe largest width to the smallest width of any deformed core shapewithin the fused and tapered bundle. This value is 1.0 if the core isnot deformed, and generally low propagation losses are observed if DRremains below 2.0.

[0031] Once a fused bundle with DOF of 1 has been fabricated, theobjective is to efficiently transfer power from the input fibers into anoutput fiber, the cross-sectional areas of the input and output elementsare matched as closely as possible, since the etendue can not beincreased or decreased by further tapering of either side. Efficientpower transfer can be enhanced by:

[0032] i) matching the fiber/bundle diameter on both sides, either bycontinuing to taper the input bundle side, or by pretapering the outputside;

[0033] ii) fusion splicing the two sides together;

[0034] iii) optionally recleaving and resplicing the two sides veryclose to the previous splice, using the surface tension during thesuccessive fusion splices to assist in matching the shapes between thetwo ends;

[0035] iv) optionally post tapering the region of the fusion splicedjunction.

[0036] The etendue of a fiber bundle cannot be reduced by merelytapering the fibers to match the size of the output fiber—thereforelimiting low loss devices to those where the etendue of the input fiberbundle is less than or equal to that of the output fiber before fusingand tapering. It has been found that the preferred manner in which toreduce the etendue of the input fiber bundle is to remove interstitialspace between the input fibers and obtain a circular bundle perimeter,which requires extensive fusing. In practice the pulling process used totaper the input fiber bundle facilitates fusing, often leading to bundletaper diameters smaller than the output fiber diameter. Therefore, apretapered output fiber 28, as illustrated, is used to minimize lossesby matching the diameter of the fully fused fiber bundle. An outputtaper ratio of final diameter to initial diameter of ˜0.6, and a taperlength of ˜3 mm are currently used. In a presently preferred embodiment,the tapered region 28 of the output fiber 24 has its polymer claddingremoved, exposing the tapered region 28 to air.

[0037] As an example, consider the case of three input multimode stepindex fibers, each with 105 micron core diameter, 125 micron claddingdiameter, and 0.22 NA, as shown in FIGS. 3a-3 c. An unfused, untaperedbundle of these fibers will have an overall diameter of 269 microns, andan effective NA of 0.22, as shown in FIG. 3a. The circumscribed area isA=Ax=56,855 μm² for a degree of fusion of 0. The etendue of the bundleis 8,642 μm²-str. Tapering the bundle will not change the etendue.Removing the interstitial space to a DOF=1 without removing thecladding, as shown in FIG. 3b, results in a final diameter of 216microns with an effective NA of 0.22 for an etendue of 5,572 μm²-str, ora 36% improvement over the unfused bundle. By completely removing allthe cladding and again eliminating interstitial space to a DOF=1, asshown in FIG. 3c, results in a final bundle diameter of 182 microns andan effective NA of 0.22, for an etendue of 3,956 μm²-str. Thisrepresents an additional 18% improvement, and is the lowest possibleetendue for the such a fiber bundle.

[0038] In a second example, as shown in FIGS. 4a and 4 b, ten inputfibers are formed into a bundle. There is more than one way to arrangethe fibers. The base area of ten fibers with 125 micron diameter is12,718 μm². FIG. 4a shows the natural hexagonal closed packedconfiguration, which is often assumed to be the optimum packingconfiguration. The area of the circle around the fibers is 271,547 μm²,resulting in a PE of 2.2. FIG. 4b shows an 8 around 2 configuration.This configuration is contained by a smaller circular diameter beforefusing, having an area of 187,805 μm², for a PE of 1.5. While bothconfigurations can fuse to a circular bundle with DOF=1, theconfiguration of FIG. 3b, because of its smaller PE, will more easilyform a circular bundle and will result in a fused bundle with lowerdeformation of the cores.

[0039]FIG. 2 shows a cross-sectional view at the splice 29 (as shown inFIG. 1) of an exemplary 7-1 input fiber bundle. The input fiber consistsof seven 125 micron multimode fibers. The input fiber bundle is taperedover the tapered region 22, such that at the splice 29 the fibers aretight-packed, and heavily fused. This results in the individual fibersundergoing induced deformation such that the cores 31 a and 31 b deformfrom their initial circular shape, and the cladding 32 surrounding thefibers is amalgamated and redistributed to form an overall circularshape. Typically, the input fiber bundle taper length over the taperedregion 22 will be ˜3 mm, and the ratio of the diameter of the cleavedend to the initial diameter of the bundle will be ˜0.25. This is shownin actual cross-sections of tapered fiber bundles in FIGS. 6a-6 f. FIGS.6a-6 f also demonstrate the absence of interstitial spacing between thefibers and near circular cross section achieved by the method of thepresent invention for two, three, four, eight, ten and sixteen fibers.

[0040] In addition to using multimode input fibers, one or moresinglemode fibers can be incorporated into the bundle. For example, ifit is desired to couple multimode pump light into the inner cladding ofa multiclad fiber while simultaneously coupling singlemode light out ofor into the singlemode core of the same multiclad fiber, the centralfiber 29 b can be a singlemode fiber. Such simultaneous singlemode andmultimode coupling to a multiclad or cladding-pumped fiber facilitatesthe transfer of signal and pump power to a cladding-pumped fiber for theconstruction of devices such as cladding-pumped fiber amplifiers.Similarly, a central fiber that couples multimode light, or only alimited number of modes into, or out of, a multiclad or cladding-pumpedfiber can be used.

[0041] A tapered fiber bundle according to the present invention can bemanufactured as follows. First, individual input fibers are arranged ina predetermined configuration and are decoated. The glass cladding ofthe input fibers can be left on the fibers and incorporated into thetapered fiber bundle, or, optionally, the cladding can be removed orpartly removed prior to tapering. Typical multimode fibers have a 100micron diameter pure silica core, surrounded by a 125 micron outerdiameter fluorine doped silica cladding, and a numerical aperture of0.22. A rotatable fixture, can be used to maintain the fibers in thepredetermined configuration. Exemplary configurations for forming inputfiber bundles with from two to twenty input fibers are shown in FIG. 5.The illustrated configurations have been found to provide the desiredtight packed input bundle, and to ensure minimal deformation of theinput fibers. It should be noted that the optimum initial configurationof fibers in a bundle does not necessarily imply “hexagonal closepacking” or “maximum number of adjacent fibers”, but is the orientationthat minimizes etendue by arranging the fibers in the smallest circle(assuming the output fiber is circular). These optimum configurationsare achieved by correct alignment of tension controlled fibers prior tofusing and tapering using the patterns in FIG. 5, leading to the fusedexemplary geometries shown in FIGS. 6a-6 f.

[0042] The bundled fibers are twisted which applies normal compressiveforces between the fibers under tension. The twisted bundle is typicallybonded on both sides to form a mechanically stable structure. The fiberbundle can, for example, be bonded with a suitable adhesive, away fromthe heat of the fusion process. The bonding prevents individual fibersfrom shifting relative to each other in subsequent steps, and maintainsdesired geometrical and physical stability. During the twisting andbonding, a precisely controlled longitudinal tension is applied to eachinput fiber to maintain the desired positioning geometry. Tensioning isin the range of from 5-20 gm.

[0043] The twisted and bonded bundle is then heated and pulled to form aheavily fused, tight-packed, tapered fiber region where the individualfibers, including their cores, are deformed. Thus, interstitial spacesbetween adjacent fibers are reduced or eliminated and the bundle isclose to circular in cross section. The heavily fused fiber bundle isthen cleaved to expose a cleaved end. In a presently preferredembodiment, the fiber bundle is cleaved near the center of the fusedarea.

[0044] The diameter of the bundle is measured and the output fiber ispretapered to match the bundle size. The bundle and pretapered outputfiber are then fusion spliced. Optionally, the bundle is recleaved about100-300 microns from the splice. The two ends are respliced together tobetter form a circular cross-section. Surface tension causes the cleavedend of the input fiber bundle to approximate the shape and size of theoutput fiber. This splicing and cleaving can, optionally, be repeateduntil the end of the input fiber bundle more precisely approaches adesired circular cross-section.

[0045] Optical fiber devices, such as optical couplers, combiners,splitters, fiber lasers and optical amplifiers, can then be formed byfusion splicing the cleaved end of the input fiber bundle to an outputfiber, such as a multimode, multiclad, or cladding-pumped fiber. In thecase of a multiclad or cladding-pumped output fiber, light transferefficiency between the core of singlemode fiber in the input bundle andthe corresponding singlemode core of the output fiber can be enhancedprior to fusion splicing by heating the fiber to cause diffusion betweenthe core and cladding, thereby expanding the mode field of thesinglemode core. This will help compensate for the expanded mode fieldcaused by tapering the corresponding singlemode core in the bundle.Furthermore, when splicing a singlemode fiber (in a bundle) to thecorresponding core of a multiclad fiber, high precision core alignmentis necessary. The tapered fiber bundle of the present invention can alsobe spliced to an output tapered fiber bundle to form a coupler, or canbe coupled to a bulk optical device (such as a solid-state laserelement) if desired. A splitter can be formed by using a combiner in thereverse direction, or by using a coupler with all except one inputterminated.

[0046] The resulting optical fiber device can be post tapered to improvemode matching between the input fiber bundle and the output fiber. Thejunction area between the tapered fiber bundle and the coupled outputscan also be recoated with a suitable coating material, such as polymeror metallic coatings.

[0047] Optical fiber devices according to the present invention havenumerous advantages over conventional optical devices. Insertion lossesare reduced and coupling efficiency improved due to the degree ofcross-sectional match between the input fiber bundle and output fiber,the degree of fusion of the bundle, and the precision of the splicebetween the input bundle and the output fiber. At high powertransmission, low loss is a critical feature as optical losses willgenerate heat in the device.

[0048] The above-described embodiments of the present invention areintended to be examples only. Alterations, modifications and variationsmay be effected to the particular embodiments by those of skill in theart without departing from the scope of the invention, which is definedsolely by the claims appended hereto.

What is claimed is:
 1. A tapered optical fiber bundle, comprising: aplurality of input fibers formed into a fiber bundle, the fiber bundlebeing adiabatically tapered, and heavily-fused into an inducedcross-sectional shape with minimally deformed cores and no interstitialspace between the input fibers.
 2. The tapered optical fiber bundle ofclaim 1, wherein the input fibers are any of multimode, single mode,multiclad and cladding pumped fibers.
 3. An optical fiber device,comprising: a tapered fiber bundle having a plurality of input fibers,adiabatically tapered, and heavily-fused into an induced compact shapewith minimally deformed cores and no interstitial space between theinput fibers at a cleaved end; and an output element coupled to thecleaved end.
 4. The optical fiber device of claim 3, wherein the outputelement is another tapered fiber bundle.
 5. The optical fiber device ofclaim 3, wherein the output element is a single optical fiber.
 6. Theoptical fiber device of claim 5, wherein the single optical fiber is amultimode optical fiber.
 7. The optical fiber device of claim 6, whereinat least one of the input fibers is terminated to reduce backreflections.
 8. The optical fiber device of claim 5, wherein the singleoptical fiber is a double clad fiber.
 9. The optical fiber device ofclaim 5, wherein the single optical fiber is pre-tapered.
 10. Theoptical fiber device of claim 3, wherein the output element is fusionspliced to the cleaved end.
 11. The optical fiber device of claim 9,wherein a spliced junction between the tapered fiber bundle and theoutput element is post-tapered.
 12. The use of the optical fiber deviceof claim 3 as any one of an optical combiner, an optical splitter, acladding-pumped fiber laser, and a cladding-pumped optical amplifier.13. A method of manufacturing an optical fiber device, comprising: i)positioning a plurality of optical fibers in a predeterminedconfiguration for forming an optimized encircling radius; (ii) bundlingthe positioned plurality of optical fibers while controlling the tensionapplied on individual fibers to result in a fiber bundle with minimizedoverall diameter; (iii) heating and pulling the fiber bundle to heavilyfuse the fiber bundle in an adiabatically tapered region into an inducedshape with no interstitial space between fibers.
 14. The method of claim13, further including twisting the positioned plurality of opticalfibers.
 15. The method of claim 13, wherein positioning further includesbonding the plurality of optical fibers with an adhesive to secure theirpositions before fusing and tapering.
 16. The method of claim 13,further including cleaving the bundle at the tapered region.
 17. Themethod of claim 16, where the cleaved end of the bundle is reshaped intoa desired cross-section by, at least once, fusion splicing the cleavedend to an optical fiber to match its cross-sectional geometry, andre-cleaving the optical fiber bundle.
 18. The method of claim 16,further including coupling the cleaved end to an optical system.
 19. Themethod of claim 16, further including fusion splicing the cleaved end toone of a single optical fiber and an output tapered fiber bundle. 20.The method of claim 19, further including pre-tapering of the singleoptical fiber.
 21. The method of claim 19, further includingpost-tapering of a junction between the tapered fiber bundle and the oneof the single optical fiber and the tapered fiber bundle.
 22. The methodof claim 13, further including at least partial removal of cladding fromthe plurality of optical fibers.
 23. The method of claim 19, furtherincluding at least partial removal of cladding from the one of theoutput optical fiber and the output tapered fiber bundle.
 24. The methodof claim 19, further including re-coating at least part of a junctionbetween the tapered fiber bundle and the one of the output optical fiberand the output tapered fiber bundle with a coating material.
 25. Themethod of claim 24, wherein the coating material is one of polymer andmetallic coatings materials.
 26. The method of claim 19, furtherincluding heating of the one of the single optical fiber and claddingpumped fiber to diffuse its core to improve the transfer of light from acorresponding singlemode fiber in the input bundle.
 27. A star coupler,comprising: a tapered fiber bundle formed in the midsection of aplurality of fibers, adiabatically tapered, and heavily-fused into aninduced compact shape with minimally deformed cores and no interstitialspace between the fibers, such that the plurality of fibers form inputand output leads on each side of the fused bundle.
 28. The star couplerof claim 28, wherein at least one of the input leads is terminated toreduce back reflections.